Pultrusion is a well known technique for forming composite structures. In general, pultrusion involves the steps of unwinding a plurality of endless reinforcements, collating the reinforcements into a layered arrangement, wetting and/or saturating the reinforcements with a resin, and transporting the layered arrangement through a pultrusion die wherein the cross-sectional shape is formed and the resin cured.
Structural components in the form of beams, ribs, “J” stiffeners, “C” channels and “I” beams lend themselves well to the pultrusion process of manufacture. The strength to weight ratio of composite structural materials is many times higher than alloys of metal. This has led to extensive use in the aerospace industry. Other industries are expected to benefit as further improvements in laminated composites are made available.
Most typically the reinforcements are fabrics or fiber tows of graphite, fiberglass, Kevlar, and the like. The reinforcement is typically chosen based on strength and weight and the ability of the particular reinforcement to be wet by the resin of choice. Maximum strength is achieved, under some circumstances, when the resin completely saturates the reinforcement such that the final cross-section of the composite is a continuous polymerized resin with bands of reinforcement layered therein. If the resin fails to thoroughly wet, and saturate, the reinforcement the strength of the composite is compromised. In this instance the cross-section is discontinuous since there are regions which are void of polymerized resin or which have insufficient polymerized resin to achieve maximum strength. For a given choice of resins there is a limited choice of materials which can form the reinforcement. Conversely, for a given reinforcement choice the resin must be selected which will sufficiently saturate, or strongly adhere to, the reinforcement.
The properties of a resin must also be compatible with the demands of the pultrusion process. The pot life must be sufficient to allow a sufficient length of composite to be manufactured without premature curing or aging out. The resin must also be curable in a reasonable period of time. It is most preferred that the resin can be cured during the residence time in the pultrusion die to avoid relaxation or running of the resin after exiting the die. If the curing time is long the rate at which the reinforcement can be transported through the die is decreased and productivity of the manufacturing facility becomes unattractive and cost of the composite increases. Polyester resins, vinyl esters, urethanes and epoxy resins are known to be compatible with the pultrusion process but these resins have not exhibited the mechanical properties which are suitable for many usages or are limited in their ability to wet and saturate some reinforcement materials thereby limiting their use.
Exemplary pultrusion methods, materials and techniques are provided in U.S. Pat. Nos. 5,989,376; 5,176,865; 5,084,222; 4,338,363; 5,556,496; 4,754,015; 4,861,621 and 4,842,667.
It has been a long standing goal to expand the composite structures which can be achieved with the pultrusion processes. In many cases, this goal has been thwarted by the limited choice of resins available. It is one object of the present invention to provide a pultrusion process with new resins which can expand the properties which can be achieved with composite materials and the applications wherein they can be incorporated.